Heated Metal Oxide Sensors
There is a large background of information regarding metal oxide chemical sensors. These have all been on heated substrates or measured by electrochemical detection. For example see Eranna, G. et al. “Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review” Critical Reviews in Solids State and Materials Sciences, 29: 111-188, 2004 (herein denoted in Ref. 1) and the references therein.
It is desirable to provide sensors for detecting ambient levels of gases, particularly noxious gases. Heated metal oxide sensors are well studied in the literature. As a recent review, Eranna et al, (table 23, page 174) shows the range of gases that can be detected and the metal oxides that are sensitive to each gas.
Many metal oxides are semiconducting. This means there is an energy gap between the population of electrons called the valance band and the conduction band where these electrons can move through the material. This energy gap is commonly called the band gap and denoted as Eg. The metal oxide sensors take advantage of this semiconducting nature by promoting electrons from the valence band to the conduction band through heat. This thermal excitation of electrons can change the surface energy of the metal oxide promoting facile chemical reactions.
Metal oxides typically have an amorphous crystal structure. This means there will be individual crystalline grains but that there is no long-range order to the surface. The interface between each crystallite creates a grain boundary. Conduction through a metal oxide is limited by the energy barrier created at each grain boundary. In addition to the change in surface energy heat changes the energy level of barriers created at these individual grain boundaries.
Carbon monoxide poisoning presents a major problem in civilian and military sectors. It is estimated that more than 500 people accidentally die from carbon monoxide (“CO”) poisoning each year in the United States, more than from any other poison. In addition, an estimated 10,000 people are treated annually for symptoms of CO exposure. While most of the household CO related incidents could be identified and treated, the situation is more critical in aircraft environments where a lack of suitable monitoring devices is available.
Carbon monoxide has about 210 times the affinity to bind to hemoglobin compared to oxygen. CO is an odorless, tasteless, colorless gas that causes hypaemic hypoxia wherein there is a reduced oxygen carrying capacity of the blood. Carbon monoxide in the blood creates carboxyhaemoglobin (COHb) which prevents oxygen uptake. At sea level, increased levels of COHb cause various symptoms ranging from headache to unconsciousness. At 200 ppm, CO at sea level causes a headache (equivalent to 15-20% COHb content in the body). At higher altitudes, the effects of CO poisoning and altitude hypoxia are cumulative, driving a need for a continuous low-level monitoring of sub-200 ppm levels of CO in aircraft cabins.
Several metal oxide and electrochemical sensors have been operational in household CO detection alarms over the past decade, but none have had the precision to continuously and accurately measure lower ppm levels of CO. Continuous monitoring of carbon monoxide at 35 to 200 ppm levels presents a challenge to any commercially available CO detector technology. Continuous carbon monoxide monitoring is critical in the household, industrial and military sectors. At present, three technologies are used in the manufacture of carbon monoxide alarms. The advantages and disadvantages of each method are outlined below.
Heated Metal Oxide Based Detectors for Carbon Monoxide (CO)
Semiconductor based sensors use heated tin dioxide thin films on a ceramic substrate. CO is oxidized on the high temperature surface. The current increases as the tin dioxide is exposed to carbon monoxide. Microchip controlled electronics detect the change in current and will sound an alarm when levels of CO, as measured by the current, exceed a defined threshold. These sensors operate at high temperatures, greater than 400° C., contributing to high power consumption. This high temperature makes them susceptible to false signals generated by chemically similar analytes. The following is an advantage: inexpensive and easy to produce. The following are disadvantages: high power consumption, slow cycle time; oxygen contamination; susceptible to false positive signals; and requires heating to regenerate system. The above outlined technology remains insufficient to present a complete solution for the continuous detection of carbon monoxide in the lower ppm ranges.
Molybdenum oxide (MoO3) thin films prepared by sol-gel and RF magnetron sputtering processes were previously employed in the development of CO sensors' as described in “Carbon Monoxide response of molybdenum oxide thin films deposited by different techniques,” by E. Comini, G. Faglia, G. Sberveglieri, C. Cantalini, M. Passacantando, S. Santucci, in Sensors and Actuators B 68, pp. 168-174 (2000), denoted herein Reference 2. The RF deposited films had a needle-like structure with longitudinal dimension ranging from 200-400 nm. The response was measured by applying a constant potential of 1 V to the sensing layer and registering the resistance with a picoammeter. This CO sensor operates as a chemiresistor. FIG. 8 of Reference 2 shows the dynamic responses of a sol-gel sensor and an RF sputtered sensor at 300° C. to a square concentration pulse of 30 ppm CO. The current changes shown in FIG. 8 of Reference 2 are the picoamp range. This range has the disadvantage that it tends to be almost impossible to record such weak output for a continuous monitor without the use of non-portable, highly sophisticated equipment. The sensor in Reference 2 was operated at 300° C. Heating a sensor substrate consumes much electrical power. This is a disadvantage for a portable device.
Further, research has been continuing in methods of depositing metal oxides. For example, the authors of “Size-selective electrodeposition of meso-scale metal particles: a general method,” by H. Liu, F. Favier, K. Ng, M. P. Zach, R. M. Penner, in Electrochimica Acta 47 pp. 671-677 (2001), denoted herein Reference 3, demonstrated that monodisperse nanoparticles of molybdenum dioxide can be grown on a conductive surface using a pulsed voltammetric technique. FIG. 5 of Reference 3 shows the scanning electron micrograph of molybdenum dioxide metal nanoparticles on graphite basal plane surfaces. As shown in that Figure, the nanoparticles have with an apparent size, as indicated by a 1 micrometer scale line, of 100-200 nm. It is also possible to oxidize an existing metal film.
Notwithstanding the above teachings, there is a strong requirement for an alternative technology to heated metal oxide sensors. In particular, there remains a need for gas sensors having low power requirements, broad environmental operating range, fast response time, high selectivity and high sensitivity.